Ruthenium tetroxide (RuO.sub.4) is reduced to rethenium oxide with a low valence and becomes black when brought into contact with organic materials, particularly oils. For this nature, when brought into contact with latent fingerprints, ruthenium tetroxide comes into contact with oils and fats, or proteins contained in organic materials in the fingerprints, and is converted into ruthenium oxide with a low valence, whose color becomes black, and is attached to the latent fingerprints. Therefore, one can distinctively detect the finger mark of the fingerprint.
Ruthenium tetroxide is a solid having a melting point of 25.5.degree. C. and a boiling point of 100.8.degree. C., readily volatile at normal temperature, and is soluble in water at about 2%. Ruthenium tetroxide dissolved in water is liable to be rapidly reduced to ruthenium oxide with a low valence. Accordingly, since it is difficult to stably store an aqueous solution of ruthenium tetroxide over a long period of time, in practice, it is substantially impossible to use the aqueous solution of ruthenium tetroxide as a detector solution of latent fingerprints.
As conventional methods using ruthenium tetroxide for detecting fingerprints, there is a technology described in Robert D. Olsen, Sr., Scott's Fingerprint Mechanics, p. 309 (1978), published by Charles C. Thomas Publisher. Springfield, Ill., U.S.A. The technology described in this literature is a method in which solid ruthenium tetroxide per se is heated on a water bath to vaporize the ruthenium tetroxide, or while utilizing the property of ruthenium tetroxide that it is slightly soluble in water, a ruthenium tetroxide gas generated from its aqueous solution is brought into contact with fingerprints. However, in the above-described direct use method, the temperature at heating on the water bath is limited to not higher than 122.degree. F. (50.degree. C.), and if the heating is carried out at temperatures higher than this temperature, there is a fear of explosion. Also, in the aqueous solution method, it is described that it takes a long period of time to obtain even indistinct fingerprint images because the amount of the generated gas is low. In any of these methods, it is extremely difficult to deal with ruthenium tetroxide, and it is mentioned that these methods must be used only by qualified persons in the laboratories. Hence, these methods have not yet been put into practical uses.
As described above, since it is substantially impossible to practice the use of ruthenium tetroxide which have already been produced for detecting latent fingerprints on the spot of the criminal investigation, a method for detecting latent fingerprints was invented by the present inventors for the purpose of practicing the use on the spot of the criminal investigation and said method has been published (see Japanese Patent Laid-Open No. 3-29642). The gist of this latent finger print detection method is to bring a ruthenium tetroxide gas in the nascent state into contact with latent fingerprints. The method described in the above-described publication as a specific method for using a ruthenium tetroxide gas in the nascent state comprises mixing an aqueous solution of ruthenium chloride with an aqueous solution of ceric ammonium nitrate. However, the above-described method also involves some defects as described below.
(1) The two kinds of the aqueous solutions must be carried to the spot of the fingerprint detection. PA1 (2) The two kinds of the aqueous solutions must be mixed with each other on the spot of the fingerprint detection. PA1 (3) In many cases, the range (area) with which the ruthenium tetroxide gas is brought into contact must be limited. PA1 (4) In the case that the ruthenium tetroxide gas is brought into contact with a wider area, large quantities of the two kinds of the solutions to be mixed must be used so that the method lacks in simplicity. PA1 (1) A ruthenium tetroxide solid is dissolved in a solvent. PA1 (2) Ruthenium tetroxide is generated (synthesized) and absorbed in a solvent. PA1 (3) The generation and absorption of ruthenium tetroxide are carried out at the same time in the same instrument. PA1 (A) A solid of ruthenium tetroxide is heated on a water bath to form a gas, which is then introduced into a solvent and absorbed (dissolved) therein. PA1 (B) A ruthenium aqueous solution (such as, for example, a ruthenium chloride aqueous solution and a ruthenium nitrate aqueous solution) is mixed with a ceric ammonium nitrite aqueous solution to generate ruthenium tetroxide, the gas of which is then introduced into a solvent and absorbed therein. Also, as described above, depending on the boiling point of the solvent, the absorption method requires or does not require the cooling.